The invention relates to a method and arrangement for continuously measuring substances in a pressurised body fluid. A probe (202) for enabling continuous measurement of the concentration of one or more substances in a pressurized body fluid is provided. The probe is adapted to be inserted into the pressurized body fluid, and comprises an interface and an outlet lumen. Furthermore, the probe is configured so that a continuous and spontaneous fluid flow is established from the body fluid through the interface to the outlet lumen. The fluid flow from the outlet may be analyzed by a sensor.
